Spectroscopy of Arcturus

[athornett]

Hi All,

The following images are of my analysis of a spectrum of Arcturus I took using my CCDSPEC spectrometer. On this occasion, I used my Sky Watcher Equinox Pro 120mm OTA on EQ6 Pro (although I was hand guiding it rather than using the drives). The camera is a QHY6 and the acquisition software was EZCAP which comes with the camera. I took the spectrum in Lichfield, Staffordshire, UK on 4/8/2018 and analysed it 26/9/2018 using RSPEC software.

Attached are:

1. Uncalibrated spectrum line graph (x-projection).

2. Calibrated spectrum of Arcturus - I used an amateur spectrum on the internet to provide three data points for the RSPEC Calibration Wizard (linear approximation).

3. A plot of my calibrated spectrum against the closest reference spectrum I could find in RSPEC. Arcturus is spectral type K1.5IIIFe-0.5 but closest match I could find on RSPEC was K1iv, so I have plotted against that. In spite of the slight differences, I have been able to identify almost exact matches for range of lines between the two spectra - it amazes me how amateurs can obtain incredibly precise data using spectroscopes on their very modest backyard setups! The vertical white lines on this graph are several points where peaks on both graphs occur at the same point. There are several others that I have not marked that also clearly match.

4. From this I have generated a calibration graph for my own future use which I have also posted here - if you have a better/alternative one you have created please do upload it in response to my post here.

Andy

 

[robin_astro]

Hi Andy,

You cannot use stars like Arcturus for wavelength calibration because the features you have identified are actually blends of many lines. (Arcturus is a K star with many hundreds of closely spaced metal lines which are unresolved at this resolution. )

You can find a spectrum of Arcturus  here taken at high resolution by the UVES spectrograph on the VLT showing how many lines there are.  (You can zoom in to see more detail)

 http://www.eso.org/sci/php/tools/uvespop/bin/readspectra.cgi?star_ID=124897&star_NAME=ARCTURUS&star_HDN=124897&out_FORMAT=gif&wave_LO=3070&wave_HI=8540&sa=Plot

A better procedure if you want to use a star for wavelength calibration is to first calibrate using a hot eg A type star which have clear well resolved H Balmer lines with known wavelengths as you did for Vega. You can then use this calibration for any target.  (You can also use this hot star spectrum to calibrate response of  your spectrograph and the effect of the atmosphere so your spectra will match the shape of the published spectra)

Robin

[robin_astro]

6 minutes ago, robin_astro said:

You cannot use stars like Arcturus for wavelength calibration because the features you have identified are actually blends of many lines. (Arcturus is a K star with many hundreds of closely spaced metal lines which are unresolved at this resolution. )

For example, here is the  UVES spectrum of Arcturus  zooming in on the region of the feature you have labelled 5810A.

Robin

[athornett]

Hi Robin, this is very interesting. What is the best resource for standard spectra showing the main lines for stars such as Vega or other groups so that I can use them for calibration or just check my spectra against them? So far I have just downloaded whatever I can find on Google Images.

Andy

[Merlin66]

All the amateur spectral processing packages (ISIS, BASS Project, VSpec etc.) have built -in data bases for the stellar classifications - Pickles (or Miles). After calibration, you can bring up a comparison spectrum to see/ match the lines.

 

Richard Walker's "Spectral Atlas" (CUP) has fully annotated spectra for most/ all the stars the amateur observes. Recommended.

https://www.cambridge.org/gb/academic/subjects/physics/amateur-and-popular-astronomy/spectral-atlas-amateur-astronomers-guide-spectra-astronomical-objects-and-terrestrial-light-sources?format=HB&isbn=9781107165908

 

 

[robin_astro]

2 hours ago, athornett said:

Hi Robin, this is very interesting. What is the best resource for standard spectra showing the main lines for stars such as Vega or other groups so that I can use them for calibration or just check my spectra against them? So far I have just downloaded whatever I can find on Google Images.

Andy

Hi Andy,

Vega is spectral type A0v.  You can call up the spectrum in RSpec etc. The obvious strong lines  are all from the Hydrogen Balmer series which are at known wavelengths.  

https://en.wikipedia.org/wiki/Balmer_series 

A good source of spectral classifications for almost a million  stars  is this database by Brian Skiff. 

http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/mk

You can then call up that spectral type from the (Pickles) library in RSpec etc to get an idea of what it should look like

 

Robin